The NPR has produced a nice visualization / video showing how population grew to 7 billion (original article):

If you want to model the improvement in child survival, you just turn the birth tap off (or nearly). Then, with wealth, prevention, healthcare and better food, the population will also grow older (death tap also turned off or nearly) and during a certain time, lots of adults will be economically active (i.e. they will work and consume). This is a demographic dividend. But it comes with a risk: at the next stage, there might be a disproportionately high number of people compared to / depending on a small number of active adults (the next generation). In addition, if you fill it up slowly but you also empty it slowly, the container risk to be full soon, it all depends on the various rates …

Note that this representation is also very effective to understand the basics of compartmental models in epidemiology 🙂

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